Foods such as confectionery are generally crisp and good in taste if they include voids therein and, therefore, it has commonly been carried out to intentionally form voids in these foods. For instance, Japanese Un-examined Patent Publication No. 52-21368 discloses a method for preparing a dough for use in making baked confectionery which comprises kneading a dough maintained in a closed condition while injecting compressed gas therein and then releasing the pressure to ordinary pressure to obtain a dough having a lot of voids uniformly distributed therein. More specifically, in the Un-examined Publication it is disclosed that the dough is subjected to gas injection at a pressure of about 4 kg/cm.sup.2 and that the release of the pressure is effected about 2 minutes after the gas injection. Furthermore, Japanese Patent Publication No. 57-49182 discloses a method for producing foods of fine and porous texture which comprises freezing a solid moisture-containing food of viscous or dense texture while pressing the foods at a gauge pressure of 20 to 70 kg/cm.sup.2 using a highly compressed gas, then thawing and dehydrating the food. In addition, Japanese Patent Publication No. 58-26942 discloses a method for producing granular confectionery containing voids therein which comprises transferring liquid confectionery in a container for solidifying and grinding the liquid confectionery after forming voids therein, separating the container for solidifying and grinding which contains the liquid confectionery having voids therein from a container for forming voids in the liquid confectionery and transferring the container for solidifying and grinding to a freezing apparatus to force-cool the same, so as to enhance the freezing efficiency. However, in the foregoing methods, it is not possible to sufficiently expand foods and, in particular, properties of solid foods such as meat and vegetable are not improved even when such a method is applied to these foods.
Moreover, if voids are formed within these foods and then the foods are solidified by freezing the same according to any one of the foregoing methods, the freezing treatment is carried using equipment different from that for the pressing treatment (void-forming treatment). Therefore, these methods include the use of complicated apparatuses and procedures. It therefore takes a long period of time to cool and solidify the expanded foods.